An astigmatism method is known as a detection method for detecting a focusing error signal in an optical head device incorporated in an optical information recording or reproducing apparatus. The astigmatism method has a feature that an optical system can be integrated with an optical system for detecting a tracking error signal by a push-pull method and a differential phase detection method and an optical system for detecting a reproduction signal, as compared with a knife-edge method and a spot-size method.
On the other hand, as a method for increasing the density of an optical recording medium, land/groove recording is known in which recording and reproduction are performed for both lands and grooves of the optical recording medium. A write-once optical recording medium and a rewritable optical recording medium typically have a groove used for tracking. The lands and grooves correspond to concave portions and convex portions, respectively, when the optical recording medium is seen from a side on which light is incident.
In a case where a focus error signal is detected by an astigmatism method for such an optical recording medium for land/groove recording, a focusing error signal obtained when light is reflected from the land is different from that obtained when light is reflected from the groove. In other words, interrelationship between the defocus amount of the optical recording medium and the level of the focusing error signal is different between a case where a focused spot formed on the optical recording medium by the optical head device is located on the land and a case where that focused spot is located on the groove. Thus, in a case where the optical head device makes an access between an inner peripheral portion and an outer peripheral portion of the optical recording medium, for example, when the focused spot formed on the optical recording medium by the optical head device crosses the grooves of the optical recording medium, the level of the focusing error signal changes between the level obtained when the focused spot is located on the land and the level obtained when the focused spot is located on the groove. This change is called as a groove-crossing noise. When the groove-crossing noise occurs, a focusing-servo operation becomes unstable and therefore prevents recording and reproduction from being performed normally.
Thus, as a detection method of a focusing error signal that can suppress the above-described groove-crossing noise, a differential astigmatism method has been proposed (see Japanese Patent Laid-Open Publication No. Hei 4-168631, for example). FIG. 5 is a block diagram of a conventional optical head device that detects a focusing error signal by the differential astigmatism method. This optical head device is described in Japanese Patent Laid-Open Publication No. Hei 4-168631.
As shown in FIG. 5, this conventional optical head device is provided with a semiconductor laser 1. In this conventional optical head device, along a traveling path of laser light emitted from the semiconductor laser 1, a collimator lens 2 for collimating the laser light emitted from the semiconductor laser 1, a diffractive optical element 3b for transmitting and diffracting light incident thereon, a beam splitter 11 for transmitting a part of light incident thereon and reflecting the remaining part of the incident light toward a predetermined direction, and an objective lens 6 for converging collimated light incident thereon are provided in that order. A disc 7 that is an optical recording medium is arranged at a focus of the objective lens 6. Please note that a direction perpendicular to the sheet of FIG. 5 is a tangential direction of a track in a region of the disc 7 which is irradiated with laser light (hereinafter, simply referred to as a tangential direction), while a vertical direction in FIG. 5 is a radial direction of the region of the disc 7 which is irradiated with the laser light (hereinafter, simply referred to as a radial direction). Moreover, along a traveling path of light reflected by the beam splitter 11, a cylindrical lens 8, a lens 9, and a photodetector 10 are provided in that order. The photodetector 10 is midway between two focal lines formed by a compound lens formed by the cylindrical lens 8 and the lens 9.
FIG. 6 is a plan view of the diffractive optical element 3b. As shown in FIG. 6, the diffractive optical element 3b includes a diffraction grating 23 formed on its entire surface. A direction in which the diffraction grating 23 extends is approximately parallel to the radial direction of the disc 7, but it is slightly inclined with respect to the radial direction. The diffraction grating 23 has a pattern of straight lines arranged at equal pitches. Please note that a circle shown with broken line in FIG. 6 represents an effective region of the objective lens 6.
In the conventional optical head device shown in FIGS. 5 and 6, the semiconductor laser 1 emits laser light, and this light is collimated by the collimator lens 2. Then, the collimated light is divided by the diffractive optical element 3b into three, i.e., zero-th order light as a main beam and plus and minus first order diffracted light as sub-beams. A part of these light beams passes through the beam splitter 11 and is then focused onto the disc 7 by the objective lens 6. Then, these light beams are reflected by the disc 7. The three reflected light beams from the disc 7 pass through the objective lens 6 in the reverse direction, and a part of those light beams is reflected by the beam splitter 11. The reflected part passes through the cylindrical lens 8 and the lens 9 and is then received by the photodetector 10.
FIG. 7 is a plan view showing positions of focused spots on the disc 7. Focused spots 14a, 14d, and 14e are spots of the zero-th order light, plus first order diffracted light and minus first order diffracted light from the diffractive optical element 3b, respectively. The focused spot 14a is located on a track 13; the focused spot 14d is located on a right track adjacent to the track 13; and the focused spot 14e is located on a left track adjacent to the track 13. When the track 13 is a land, the right and left tracks adjacent to the track 13 are grooves. When the track 13 is a groove, the right and left tracks adjacent to the track 13 are lands.
FIG. 8 is a plan view showing a light-receiving portion of the photodetector 10 and positions of light spots on the photodetector 10. As shown in FIG. 8, the photodetector 10 has three square light-receiving regions 15x, 15y, and 15z each of which is formed by four light-receiving portions arranged in a matrix of 2×2. That is, the light-receiving region 15x is divided into four square light-receiving portions 15a through 15d; the light-receiving region 15y is divided into four square light-receiving portions 15e through 15h; and the light-receiving region 15z is divided into four square light-receiving portions 15i through 15l. When light transmitted by the lens 9 is incident on the photodetector 10, light spots 16a, 16d, and 16e are formed on the light-receiving regions 15x, 15y, and 15z, respectively. Each of the light-receiving portions 15a through 15l outputs an electric signal in accordance with the intensity of light incident thereon.
The light spot 16a is a spot of the zero-th order light of the diffractive optical element 3b and is received by the light-receiving portions 15a through 15d. The light spot 16d is a spot of the plus first order diffracted light of the diffractive optical element 3b and is received by the light-receiving portions 15e through 15h. The light spot 16e is a spot of the minus first order diffracted light of the diffractive optical element 3b and is received by the light-receiving portions 15i through 15l. Because of an effect of the cylindrical lens 8 and the lens 9, the intensity distribution in the light spot in the tangential direction and that in the radial direction are interchanged.
An extending direction of a boundary between the light-receiving portions 15e and 15f, a boundary between the light-receiving portions 15g and 15h, a boundary between the light-receiving portions 15a and 15b, a boundary between the light-receiving portions 15c and 15d, a boundary between the light-receiving portions 15i and 15j, and a boundary between the light-receiving portions 15k and 15l corresponds to the tangential direction of the disc 7 in the focused spots 14a, 14d, and 14e on the disc 7 shown in FIG. 7. Moreover, an extending direction of a boundary between the light-receiving portions 15e and 15g, a boundary between the light-receiving portions 15f and 15h, a boundary between the light-receiving portions 15a and 15c, a boundary between the light-receiving portions 15b and 15d, a boundary between the light-receiving portions 15i and 15k, and a boundary between the light-receiving portions 15j and 15l corresponds to the radial direction of the disc 7 in the focused spots 14a, 14d, and 14e on the disc 7 shown in FIG. 7.
Assuming that electric signals output from the light-receiving portions 15a through 15l are represented by V15a through V15l, respectively, a focusing error signal FEM of the main beam by an astigmatism method is obtained by the following expression 1 .FEM=(V15a+V15d)−(V15b+V15c)  (1)
A focusing error signal FES of the sub-beams by an astigmatism method is obtained by following expression 2.
                    FES        =                              (                          V15e              +              V15h              +              V15i              +              V15l                        )                    -                                          ⁢                                          ⁢                      (                          V15f              +              V15g              +              V15j              +              V15k                        )                                              (        2        )            
Moreover, a focusing error signal FE by a differential astigmatism method is obtained by following expression 3, where a ratio of the light amount of the main beam to the sub-beams is represented by K.FE=FEM+K×FES  (3)
FIGS. 9(a) through (c) are graphs showing exemplary various focusing error signals that were calculated, in each of which the horizontal axis represents the defocus amount of the disc 7 and the vertical axis represents the level of the focusing error signal that was normalized with a sum signal. FIG. 9(a) shows the focusing error signal of the main beam calculated by the above-mentioned expression 1; FIG. 9(b) shows the focusing error signal of the sub-beams calculated by above-mentioned expression 2; and FIG. 9(c) shows the focusing error signal by a differential astigmatism method calculated by above-mentioned expression 3. In FIGS. 9(a) through (c), a black circle (●) represents the focusing error signal in a case where the focused spot of the main beam is located on the land, while a white circle (◯) represents the focusing error signal in a case where the focused spot of the main beam is located on the groove. A condition of calculation that was set is as follows: the wavelength of the light source is 405 nm, numerical aperture of the objective lens 6 is 0.65, a track pitch (the width of the land and the groove) is 0.34 μm, and the depth of the groove is 45 nm.
When the focusing error signal is detected by a simple astigmatism method using the main beam only, as shown in FIG. 9(a), dependency of the focusing error signal on the defocus amount on the land is different from that on the groove. Thus, every time the light spot on the disc crosses the land and the groove, the groove-crossing noise is generated. On the other hand, when the focusing error signal is detected by a differential astigmatism method using the main beam and the sub-beams, as shown in FIG. 9(c), the defocus-amount dependency of the focusing error signal on the land is the same as that on the groove. Thus, generation of the groove-crossing noise can be suppressed. This is because the defocus-amount dependency of the focusing error signal of the main beam shown in FIG. 9(a) is opposite to that of the sub-beams shown in FIG. 9(b), namely, that of the land and that of the groove are opposite to each other. Thus, by adding both the defocus-amount dependency of the focusing error signal of the main beam and that of the sub-beams, the difference between the land and the groove can be canceled out. In other words, the sum of the signal level shown with the black circle in FIG. 9(a) (signal level of the main beam when the spot of the main beam is located on the land) and the signal level shown with the black circle in FIG. 9(b) (signal level of the sub-beams when the spot of the main beam is located on the land) is substantially equal to the sum of the signal level shown with the white circle in FIG. 9(a) (signal level of the main beam when the spot of the main beam is located on the groove) and the signal level shown with the white circle in FIG. 9(b) (signal level of the sub-beams when the spot of the main beam is located on the groove).
However, in order to obtain a favorable focusing error signal by a differential astigmatism method in the conventional optical head device shown in FIG. 5, it is necessary that the focused spot of the main beam be arranged away from the focused spots of the sub-beams by one track pitch in the radial direction on the disc 7. Thus, when there are a plurality of types of optical recording media each having a different track pitch, the conventional optical head device can obtain a favorable focusing error signal by a differential astigmatism method only for one of the optical recording media.
By the way, in a case of detecting a tracking error signal by a push-pull method in an optical head device, when an objective lens moves in a radial direction of an optical recording medium, offset is generated in the tracking error signal, thus preventing recording and reproduction from being performed normally. As a detection method of the tracking error signal that can suppress the above offset, a differential push-pull method has been proposed. Moreover, a differential push-pull method that can accommodate a plurality of types of optical recording media each having a different track pitch, has been proposed. Therefore, as for the differential astigmatism method, in order to accommodate a plurality of types of optical recording media each having a different track pitch, respectively, employing the same structure can be considered.
As a conventional optical head device that detects a tracking error signal by a differential push-pull method for a plurality of types of optical recording media each having a different track pitch, an optical head device described in Japanese Patent Laid-Open Publication No. Hei 9-81942 is known. In this optical head device, the diffractive optical element 3b in the conventional optical head device shown in FIG. 5 is replaced with a diffractive optical element 3c described below.
FIG. 10 is a plan view of the diffractive optical element 3c. As shown in FIG. 10, the diffractive optical element 3c is divided into two regions 12e and 12f by a straight line that passes through an optical axis of incident light and is parallel to the tangential direction of the disc 7. Each of the regions 12e and 12f has a diffraction grating 23 formed therein. A direction in which the diffraction grating 23 extends is the radial direction of the disc 7 and a pattern of the grating is formed by straight lines arranged at equal pitches. Moreover, the phase of the grating in the region 12e is shifted from that in the region 12f by π (corresponding to a half pitch). Thus, the phase of plus first order diffracted light from the region 12e and the phase of plus first order diffracted light from the region 12f are shifted from each other by π, while the phase of minus first order diffracted light from the region 12e and the phase of minus first order diffracted light from the region 12f are shifted from each other by π. Please note that a circle shown with broken line represents an effective region of the objective lens 6.
FIG. 11 is a plan view showing positions of focused spots on the disc 7. Focused spots 14a, 14f, and 14g are spots of zero-th order light, plus first order diffracted light, and minus first order diffracted light from the diffractive optical element 3c, respectively. These three focused spots are located on the same track 13. Incidentally, the track 13 is a land or groove. Since the phase in the sub-beam on the left side of a straight line, which passes through the optical axis and is parallel to the tangential direction of the disc 7, is different from that on the right side of the straight line by π, the intensity becomes zero on the boundary between the right region and the left region in each of the focused spots 14f and 14g of the sub-beams. Thus, on the right and left sides of the center line of the track 13, two peaks that are equal to each other in intensity appear.
As described in Japanese Patent Laid-Open Publication No. Hei 9-81942, as for the tracking error signal obtained by a differential push-pull method, to shift the phase of the sub-beam on the left side with respect to the straight line, which passes through the optical axis and is parallel to the tangential direction of the disc 7, from that on the right side with respect to the straight line by π by shifting the phases of the gratings in the regions 12e and 12f in the diffractive optical element 3c from each other by π has equivalent effects to those achieved by arranging the focused spot of the main beam away from the focused spots of the sub-beams by one track pitch in the radial direction on the disc 7. Moreover, since three focused spots are located on the same track in this optical head device, relative positions of the focused spots with respect to the track 13 do not depend on the track pitch. Thus, this optical head device can accommodate a plurality of types of optical recording media each having a different track pitch.
The structure of the light-receiving portions of the photodetector 10 and the arrangement of the light spots on the photodetector 10 in this conventional optical head device are the same as those shown in FIG. 8. Please note that light spots 16a, 16d, and 16e are spots of zero-th order light, plus first order diffracted light, and minus first order diffracted light from the diffractive optical element 3c, respectively. In this optical head device, focusing error signals of the main beam and sub-beams by an astigmatism method and a focusing error signal by a differential astigmatism method are calculated by the expressions 1 to 3 described above.
In addition, the inventors of the present invention developed and disclosed a technique for correctly detecting a tilt of an optical recording medium in its radial direction (radial tilt) by dividing a diffractive optical element into a plurality of regions in the optical head device (see Japanese Patent Laid-Open Publication No. 2001-307358). This technique can detect the radial tilt correctly even if an objective lens moves in the radial direction of the optical recording medium. According to this technique, even if the objective lens moves in the radial direction of the optical recording medium, no offset is generated in a radial tilt signal and therefore the radial tilt can be correctly detected.
However, the aforementioned conventional optical head device has the following problems. FIGS. 12(a) through (c) are graphs showing exemplary various focusing error signals that were calculated, in each of which the horizontal axis represents the defocus amount of the disc 7 and the vertical axis represents the level of the focusing error signal normalized with a sum signal. FIG. 12(a) shows the focusing error signal of the main beam calculated by the above-mentioned expression 1; FIG. 12(b) shows the focusing error signal of the sub-beam calculated by the above-mentioned expression 2; and FIG. 12(c) shows the focusing error signal by a differential astigmatism method calculated by the above-mentioned expression 3. Moreover, in each graph, a black circle (●) represents the focusing error signal when the focused spot of the main beam is located on the land while a white circle (◯) represents the focusing error signal when the focused spot of the main beam is located on the groove. Please note that the calculation condition that was set in this calculation is the same as that in the calculation of the focusing error signals shown in FIGS. 9(a) through (c).
As shown in FIG. 12(a), in a case of detecting the focusing error signal by a simple astigmatism method using the main beam only, the defocus-amount dependency of the focusing error signal when the focused spot of the main beam is located on the land is different from that when the focused spot of the main beam is located on the groove. Thus, the groove-crossing noise is generated. Moreover, in a case of detecting the focusing error signal by a differential astigmatism method using the main beam and sub-beams, as shown in FIG. 12(c), the defocus-amount dependency of the focusing error signal when the focused spot of the main beam is located on the land is coincident with that when the focused spot of the main beam is located on the groove around the origin. However, they are different from each other in a region other than a region around the origin. Thus, generation of the groove-crossing noise cannot be sufficiently suppressed. Especially, in a region from −1.5 μm to +1.5 μm in which the defocus amount actually fluctuates, the levels of the focusing signal on the land and that on the groove are different from each other. This is because the defocus-amount dependency of the focusing error signal of the main beam shown in FIG. 12(a) on the land and groove is not opposite to the defocus-amount dependency of the focusing error signal of the sub-beam shown in FIG. 12(b) on the land and groove in the region other than the region around the origin. Therefore, even if they are added, the difference of the defocus-amount dependency on the land and groove between the main beam and the sub-beams cannot be canceled out sufficiently.
As described above, as for a tracking error signal by a differential push-pull method, the conventional optical head device described in Japanese Patent Laid-Open Publication No. Hei 9-81942 has an effect that it is possible to obtain the correct tracking error signal that does not depend on the track pitch. However, this conventional optical head device does not have a sufficient effect as for a focusing error signal by a differential astigmatism method.
Moreover, in the optical head device described in Japanese Patent Laid-Open Publication No. 2001-307358, as for the radial tilt signal, correct signal detection can be achieved even when the objective lens shifts. However, this optical head device does not consider a focusing error signal by a differential astigmatism method and cannot achieve a sufficient effect as for the focusing error signal like the optical head device described in Japanese Patent Laid-Open Publication No. Hei 9-81942.